1. Field of the Invention
The present invention relates to an image processing circuit and, more particularly, to an image processing circuit for use in a liquid crystal display.
2. Description of the Background Art
In recent years, liquid crystal displays have been utilized in various fields and have been utilized in televisions as well as in PC monitors. However, liquid crystal displays have low response speeds, thereby including the problem of degradation of display quality due to afterimages in cases where moving images are mainly displayed thereon as in TV applications. Therefore, overdrive processing methods have been applied to liquid crystal displays, in order to increase their response speeds. Overdrive processing is a processing method for, in cases where image data is moving images, setting the voltage applied to the liquid crystal to be higher than usual if the direction of data change from the previous frame to the current frame is positive, but setting the voltage to be lower than usual if the direction of data change from the previous frame to the current frame is negative. This method can improve the display quality of moving images.
As overdrive processing commonly applied to liquid crystal displays, there is a method which calculates the amount of overdrive using a look-up table (LUT). However, this method requires an LUT provided in accordance with the number of gray-scale of image data, which has induced the problem of increases of data due to great numbers of gray-scale. Consequently, image data has been quantized with predetermined threshold values and an LUT has been applied to such quantized data to reduce the amount of data in the LUT.
Furthermore, in the case where overdrive processing is applied to a liquid crystal display, the overdrive processing is performed if image data to be displayed is a moving image, which requires determination as to whether or not the image data to be displayed is a static image or a moving image. Further, image processing for image data is disclosed in, for example, Japanese Patent Application Laid-Open No. 06-334873 (1994).
As described above, when image data is quantized with a predetermined threshold value, quantized data thereof is utilized for determining whether or not the image data is a moving image or a static image. In this case, if there is a large difference between the image data of a current frame and the image data of the previous frame (a difference greater than several gray-scale), the image data is absolutely determined to be a moving image and overdrive processing is performed thereon.
Further, if the change to the image data of a current frame from the image data of the previous frame is about a single gray-scale, but the change strides a quantization threshold value, the image data is determined to be a moving image since their quantized values are different and overdrive processing is performed thereon. Such changes about a single gray-scale may be caused by FRC (Frame Rate Control) processing, which is pseudo gray-scale expression, or by noise. Consequently, there has been caused the problem that image data which is actually a static image is determined to be a moving image and unnecessary overdrive processing is performed thereon.
If overdrive processing is performed on image data which is actually a static image and is not required to be subjected to overdrive processing, this will cause image quality degradation due to enhanced FRC processing or image quality degradation due to enhanced noise.